ca alys s
Re iew
Ca aly ic P ocesses om Biomass-De i ed Hexoses
and Pen oses: A Recen Li e a u e O e iew
Jesús Es eban 1, Ped o Yus os 2and Miguel Lade o 2,*
1Max Planck Ins i u e o Chemical Ene gy Con e sion, S i s aße 34-36,
45740 Mülheim an de Ruh , Ge many; [email p o ec ed]
2Chemical Enginee ing and Ma e ials Depa men , Chemical Sciences School,
Complu ense Uni e si y, Ciudad Uni e si a ia s/n, 28040 Mad id, Spain; [email p o ec ed]
*Co espondence: [email p o ec ed]; Tel.: +34-91-394-4164
Recei ed: 11 No embe 2018; Accep ed: 3 Decembe 2018; Published: 7 Decembe 2018


Abs ac :
Biomass is a plen i ul enewable sou ce o ene gy, ood, eed and chemicals. I ixes abou
1–2% o he sola ene gy ecei ed by he Ea h h ough pho osyn hesis in bo h e es ial and aqua ic
plan s like mac o- and mic oalgae. As ossil esou ces deple e, biomass appea s a good complemen
and e en ually a good subs i u e eeds ock, bu s ill needs he de elopmen o ela i ely new ca aly ic
p ocesses. Fo his pu pose, ca aly ic ans o ma ions, whe he alone o combined wi h he mal
ones and sepa a ion ope a ions, ha e been unde s udy in ecen yea s. Ca aly ic bio e ine ies
a e based on dehyd a ion-hyd a ions, hyd ogena ions, oxida ions, epime iza ions, isome iza ions,
aldol condensa ions and o he eac ions o ob ain a ple ho a o chemicals, including alcohols, ke ones,
u ans and acids, as well as ma e ials such as polyca bona es. Ne e heless, he e is s ill a need
o highe selec i i y, s abili y, and egene abili y o ca alys s and o p ocess in ensi ica ion by a
wise combina ion o ope a ions, ei he in-se ies o combined (one-po ), o each economic easibili y.
He e we p esen a li e a u e su ey o he la es de elopmen s o ob aining alue-added p oduc s
using hexoses and pen oses de i ed om lignocellulosic ma e ial, as well as algae as a sou ce o
ca bohyd a es o subsequen ans o ma ions.
Keywo ds:
lignocellulose; algae; bio e ine y; monosaccha ide; pla o m chemical; ca alysis;
selec i i y; s abili y; bio-based monome s; hexoses; pen oses; sol en s
1. In oduc ion
The ad en o second gene a ion bio e ine ies has opened he possibili y o access a as mass o
monosaccha ides, such as glucose, xylose, mannose, galac ose and a abinose, apa om some o hei
de i a i es such as galac u onic acid, o name he mos abundan o he lignocellulosic biomass [
1
].
A he same ime, he need o ood and eed is di ec ing esea ch in e es s owa ds he exploi a ion
o seaweeds, mic oalgae and aqua ic plan s, owing o hei abundance. Thei majo componen s a e
alginic acid, aga o ca ageenean, which could become sou ces o pla o m chemicals o he u u e [
2
].
Nowadays, seaweed p oduc ion is in excess o 25 M /yea .
In i s -gene a ion bio e ine ies, now a a comme cial scale, se e al p oblems a e due o eeds ock
sca ci y: ee suga om suga cane and suga bee , on one hand, and s a ch om co n, po a oes and
o he oods, on he o he . Suc ose annual p oduc ion amoun s o app oxima ely 170 M ,
while 88 M
s a ch is ob ained o se e al pu poses ( hough he c op o po a oes alone in 2016 eached 377 M ).
Al hough high, his amoun is s ill e y a away om he 4700 M o oil and 7200 M o coal
consumed in 2016 [
3
]. The ood e sus uel deba e canno be main ained o long, e en only
conside ing his eason. F om he mass balance pe spec i e, second gene a ion bio e ine y p ocesses,
hough less de eloped han i s -gene a ion ones, a e mo e p omising in he long e m. CO
2
ixa ion
Ca alys s 2018,8, 637; doi:10.3390/ca al8120637 www.mdpi.com/jou nal/ca alys s
Ca alys s 2018,8, 637 2 o 39
ia pho osyn hesis eaches up o 258,000 M each yea , c ea ing 447 M o e es ial and aqua ic
biomass [4].
This ype o biomass is no c ea ed o s o e ene gy and ma e ial esou ces by li ing beings, bu o
s uc u al and ep oduc i e pu poses, so i s use is much hinde ed by i s s uc u e, posing complex
echnological challenges o i s ans o ma ion. To his end, he mochemical app oaches ocus on he
p oduc ion o ca bon- ich solids, liquids and gases (biocha , biooil, and syn hesis gas, a mix u e o
CO and H
2
-) [
5
]. Though he mix u es a e complex, hei na u e is mo e simila o ossil esou ces,
and p esen e ine y and pe ochemical echnologies can be use ul, bu wi h a highe de elopmen o
lique ac ion, Fische -T opsch echnologies and as and slow py olysis p ocesses, educing byp oduc
o ma ion in all cases and educing ene gy consump ion. One o he mos p omising s a egies is
py olysis and ca aly ic s eam e o ming o p oduce H
2
om biomass [
6
]. Fu he al e na i es wi hin
he moca aly ic p ocessing in ecen yea s, hyd ogenolysis a high H
2
p essu es and ca aly ic ans e
hyd ogenolysis (in mode a e condi ions), a e being de eloped o ob ain allyl-alcohols om glyce ol,
u ans and lac ones om u u al and hyd oxyme hyl u u al (5-HMF), se e al acids and alcohols
om le ulinic acid and i s es e s and, mos in e es ingly, se e al phenolic alcohols om lignin, he less
eac i e ac ion o lignocellulosic biomass [7].
Less ene gy-in ensi e is he biochemical app oach, based on biomass ac iona ion h ough he iad
p e ea men s-depolyme iza ions (chemical and/o enzyma ic)-syn hesis (biological and/o ca aly ic),
al hough en i onmen al conce ns should be conside ed [
8
]. E en so, biological ans o ma ions
o syngas a e possible, mixing he mo- and biochemical app oaches o ecalci an biomasses [
9
].
Al hough ecalci an , biomass can be con e ed o chemicals di ec ly by a numbe o ca aly ic ou es,
combining depolyme iza ion, polyme iza ion o humins, and eac ion o se e al chemicals, wi h 5-HMF,
u u al, le ulinic acid and o mic acid as he mos p ominen [
10
]. Fo example, om eal biomass,
5-HMF can be ob ained in 11–57% w/wyields using edible biomass and up o 60% w/wi lignocellulosic
biomass was ea ed wi h acids in
γ
- ale olac one media [
11
]. Le ulinic acid can be ob ained om pu e
cellulose by using mesopo ous ca alys s esembling cellulase ac ion [
12
], eaching yields up o 51%
w/w. Simila esul s can be ob ained wi h dica ionic ionic liquids a 100
◦
C o 3 h (yield = 55%) [
13
].
Hemicellulose is he second mos abundan polyme in lignocellulosic biomass (15–30%) a e cellulose,
wi h u u al he main pla o m chemical ha can be di ec ly o indi ec ly p oduced in as amoun s [
14
].
Fu u al can be ob ained in one-po p ocesses om hemicelluloses ich in xylose, wi h yields up o 85%
when using B øns ed acidic ionic liquids (BAILs) [
15
]. Howe e , be e yields, highe ac i i ies and
less by-p oduc s can be eached when he cons i uen monosaccha ides (glucose, xylose, uc ose, e c.)
a e he aw ma e ials o 5-HMF, u u al and le ulinic acid [
16
]. F om C5 and C6 monosaccha ides,
se e al acids ( o mic, ace ic, oxalic, malonic, gluconic and gluca ic) can be p oduced as well [
17
],
while 5-HMF is he key o chemicals such as le ulinic acid, adipic acid, 1–6 hexanediol,
ε
-cap olac am,
and se e al u ans and lac ones [
17
,
18
]. Figu e 1shows he main s eps in he ac iona ion o
biomass o ob ain he monosaccha ides and low molecula mass pla o m chemicals o u he
ca aly ic/bioca aly ic p ocessing.
This e iew compiles he mos ele an aspec s o he la es esea ch ( om 2016 o he p esen
day) on ca aly ic p ocesses om C5 and C6 monosaccha ides as pla o m chemicals ob ained om
lignocellulosic ma e ials. In addi ion, epo s on algal biomass as an in e es ing s a ing ma e ial o
monosaccha ides a e also co e ed, wi h a sligh ly longe imespan gi en he no el y and eme gence o
he opic.
Ca alys s 2018,8, 637 3 o 39
Ca alys s 2018, 8, x FOR PEER REVIEW 3 o 40
Figu e 1. Chemical bio e ine y (bio/ca alys ou es) main p ocess scheme.
2. Ca aly ic S udies o he T ans o ma ion o Glucose
Ca bohyd a es cons i u e app oxima ely 75% o he annual enewable biomass, among which
cellulose is he mos a ac i e owing o i s wide a ailabili y and, mos impo an ly, o he ac i does
no compe e wi h ood p oduc s. Lignocellulosic ma e ial can unde go di e en ypes o
p e ea men o elease i s componen s, including ac iona ion by he mochemical and physical
me hods, and biological o o ganosol and o he chemical p ocedu es. Among such componen s a e
lignin, hemicellulose and cellulose, he la e being he majo componen , ci ca 45% [19–21].
Cellulose is undoub edly he mos abundan polyme in na u e owing o i being he main
cons i uen o plan cell walls. The e o e, owing o i s massi e a ailabili y, he e is an oppo uni y o
ob ain suga s om i s depolyme iza ion by hyd olysis, which leads o ob aining soluble
oligosaccha ides and, especially, glucose. Fo such hyd olysis, cellulose can unde go enzyma ic o
chemical p ocesses using mine al acids o , mo e ecen ly, solid acid ca alys s [22].
The p esen sec ion will ocus on wo ks epo ing he exploi a ion o glucose o ob ain alue-
added p oduc s and uels o enewable o igin h ough di e en ca aly ic p ocedu es. Fo he
alo iza ion o his ma e ial, wo ypes o ans o ma ions can be iden i ied:
(a) Rou es h ough 5-HMF as a building block o u he ans o ma ions o u an-based p oduc s.
Such pa hways equi e p io isome iza ion o glucose o uc ose, om which 5-HMF can be
p oduced. Figu e 2 p esen s a scheme o a numbe o p oduc s and in e media es in some cases
ha can be syn hesized ia pa hways om 5-HMF s a ing om glucose h ough isome
uc ose. Addi ionally, his igu e also shows he de i a ion o glucose om lignocellulosic
ma e ial.
(b) Reac ions o p oduc s ob ained by non-5-HMF ela ed ou es, hus a oiding isome iza ion and
dehyd a ion as he i s s eps o he co esponding con e sion. These a e schema ized in Figu e
3.
2.1. Isome iza ion o F uc ose and Reac ions o P oduc s h ough 5-HMF Rela ed Rou es
Table 1 p esen s a compila ion o wo ks in he li e a u e ha complemen se e al o he ou es
p esen ed in Figu e 2, p o iding speci ic de ails abou he p oduc s, side-p oduc s, ca alys s
employed and ope a ing condi ions, as well as he ou comes o hei applica ion.
Figu e 1. Chemical bio e ine y (bio/ca alys ou es) main p ocess scheme.
2. Ca aly ic S udies o he T ans o ma ion o Glucose
Ca bohyd a es cons i u e app oxima ely 75% o he annual enewable biomass, among which
cellulose is he mos a ac i e owing o i s wide a ailabili y and, mos impo an ly, o he ac
i does no compe e wi h ood p oduc s. Lignocellulosic ma e ial can unde go di e en ypes o
p e ea men o elease i s componen s, including ac iona ion by he mochemical and physical
me hods, and biological o o ganosol and o he chemical p ocedu es. Among such componen s a e
lignin, hemicellulose and cellulose, he la e being he majo componen , ci ca 45% [19–21].
Cellulose is undoub edly he mos abundan polyme in na u e owing o i being he main
cons i uen o plan cell walls. The e o e, owing o i s massi e a ailabili y, he e is an oppo uni y
o ob ain suga s om i s depolyme iza ion by hyd olysis, which leads o ob aining soluble
oligosaccha ides and, especially, glucose. Fo such hyd olysis, cellulose can unde go enzyma ic
o chemical p ocesses using mine al acids o , mo e ecen ly, solid acid ca alys s [22].
The p esen sec ion will ocus on wo ks epo ing he exploi a ion o glucose o ob ain alue-added
p oduc s and uels o enewable o igin h ough di e en ca aly ic p ocedu es. Fo he alo iza ion o
his ma e ial, wo ypes o ans o ma ions can be iden i ied:
(a)
Rou es h ough 5-HMF as a building block o u he ans o ma ions o u an-based p oduc s.
Such pa hways equi e p io isome iza ion o glucose o uc ose, om which 5-HMF can be
p oduced. Figu e 2p esen s a scheme o a numbe o p oduc s and in e media es in some cases
ha can be syn hesized ia pa hways om 5-HMF s a ing om glucose h ough isome uc ose.
Addi ionally, his igu e also shows he de i a ion o glucose om lignocellulosic ma e ial.
(b)
Reac ions o p oduc s ob ained by non-5-HMF ela ed ou es, hus a oiding isome iza ion and
dehyd a ion as he i s s eps o he co esponding con e sion. These a e schema ized in Figu e 3.
2.1. Isome iza ion o F uc ose and Reac ions o P oduc s h ough 5-HMF Rela ed Rou es
Table 1p esen s a compila ion o wo ks in he li e a u e ha complemen se e al o he ou es
p esen ed in Figu e 2, p o iding speci ic de ails abou he p oduc s, side-p oduc s, ca alys s employed
and ope a ing condi ions, as well as he ou comes o hei applica ion.
Ca alys s 2018,8, 637 4 o 39
Ca alys s 2018, 8, x FOR PEER REVIEW 4 o 40
Figu e 2. Exploi a ion o glucose and uc ose de i ed om biomass o alue-added chemicals ia ans o ma ion o 5-hyd oxyme hyl u u al.
Figu e 2. Exploi a ion o glucose and uc ose de i ed om biomass o alue-added chemicals ia ans o ma ion o 5-hyd oxyme hyl u u al.
Ca alys s 2018,8, 637 5 o 39
The i s eac ion be o e ob aining 5-HMF as a building block is he isome iza ion o glucose
o uc ose. LiB has been epo ed o ac as a ca alys ob aining yields o uc ose o 30.3% in only
15 min, while he con e sion amoun ed o 51.8%, wi h mannose being ob ained mainly om a side
epime iza ion eac ion. This wo k epo ed mechanis ic s udies, whe e Li
+
ca alyzed he isome iza ion
h ough he in amolecula hyd ide shi mechanism om C2 o C1 and B
-
h ough a p o on ans e
mechanism ia an enediol in e media e [
23
]. Cu sal s ha e also been used o his eac ion, ob aining
much lowe con e sions and yields, which we e dependen on he pH o he eac ion medium.
A alues o pH = 5.3–5.5, he de ec ed species o he Cu sal s was Cu(OH)
+
, iden i ied as he Lewis
acid ac i e species, which igge s an in amolecula 1,2-hyd ide shi [24].
Fu he o sole isome iza ion o uc ose, he syn hesis o me hyl uc osides has also been
s udied by e he i ica ion. A s udy combining di e en zeoli es as supplie s o Lewis acidi y and
Dowex 50WX8-100 (B øns ed acidi y) was made eaching yields o 72% in 1 h. In he mechanis ic
s udies comple ed in his wo k i was p o en ha when ca alys s wi h B øns ed acidi y we e
employed, me hyl glycosides p e ailed, whe eas when Lewis acidi y was highe , isome iza ion
was p edominan [
25
]. In a di e en wo k, Lei ne e al. used kaolin o conduc his eac ion,
whose ad an age is i s wide a ailabili y and inexpensi e p ice. The s udy ocuses on he sc eening o
di e en kaolin samples, among which S em-2008 is he mos ac i e, and, hen, he op imiza ion o
condi ions using a ional expe imen design in con inuous low [26].
As men ioned abo e, 5-HMF is he building block o ul e io eac ions, and i has also been
sough a e as an end p oduc . To gene a e 5-HMF, he eac ion s eps equi e he dehyd a ion o
uc ose a e p io isome iza ion o glucose. Glucose ans o ma ion was epo ed using o ganic
ca alys s ea u ing basic, B øns ed acidic o bo h ypes o unc ional g oups. The basic g oups ha e
been ound o isome ize glucose o uc ose and B øns ed acidic g oups ca alyze he dehyd a ion.
Thus, sul anilinic acid pe o med be e han he o he ca alys s es ed, eaching 44% yield o 5-HMF
wi h 90% con e sion o glucose. In addi ion, mechanis ic s udies disclosed ha isome iza ion o
uc ose was ound o be he a e-limi ing s ep du ing he eac ion [
27
]. Tin phospha es, whose ac i e
si es appea o be he e acoo dina ed Sn
4+
cen e s, ha e also been employed o his eac ion
[Emim][B ] ionic liquid as sol en eaching yields up o 58.3%. The syne gis ic e ec o his ionic liquid
appea s o ha e an in luence in he high yields epo ed [
28
]. Cui e al. p oposed he p epa a ion o a
ca alys based on chi osan nanopa icles doped wi h C ions, which we e p epa ed om adso p ion
om aqueous solu ions and can he e o e ha e an applica ion in he ea men o was ewa e o
emo al o hese species. The con e sion epo ed was abou 92% and he yields o 5-HMF epo ed as
high as 64.7% [
29
]. Finally, making use o 5-sul oisoph halic acid as ligand, a in po ous coo dina ion
polyme was syn hesized on polydopamine-coa ed MnO
2
. This ca alys eached con e sions o glucose
abo e 90% and yields o 55.8%, showing good ecycling capaci y as no g ea ac i i y loss was obse ed.
Th ough 5-HMF, by di ec one-po con e sion o glucose, 2,5-dime hyl u an (2,5-DMF) was
ob ained ia hyd ogenolysis, lea ing o he u an compounds as side p oduc s. Fo his eac ion,
ca alys s based on Pd suppo ed on a Z -based me alo ganic amewo k deposi ed on sul ona ed
g aphene oxide was used, which ob ained a yield o he desi ed p oduc o 45.3% in 3 h [30].
2,5-Di o myl u an (2,5-DFF o DFF) is a enewable pla o m chemical wi h mul iple p ospec i e
applica ions in polyme , ag ochemical and pha maceu ical indus ies. This compound is ob ained
wi h good yields om uc ose (74% in a one-po dehyd a ion-ae obic oxida ion p ocess) and 5-HMF
(94%) using 3D lowe -like Ce
−
Mo mic o/nano composi e oxides wi h se e al a ios o Ce o Mo [
31
].
Likewise, in a one-po p ocess, uc ose has been con e ed in DFF by using phosphomolibdic acid
ixed in he me al–o ganic amewo ks MIL-101, eaching a yield o 75.1% [32].
Me hyl lac a e can be syn hesized h ough e o-aldol agmen a ion o 5-HMF ollowed by
dehyd a ion, ace aliza ion and isome iza ion. Yields o 47.7% we e epo ed using ZnCl
2
a e 3 h and
a kine ic analysis o he o e all ans o ma ion o glucose o me hyl lac a e wi h his ca alys e ealed
ha he ac i a ion ene gy is 117.5 kJ mol
−1
[
33
]. In a simila s udy, Sn-Be a zeoli e was used, achie ing
sligh ly lowe yields o 43% o p oduc in a longe pe iod gi ing uc osides as by-p oduc s. In ac ,

Ca alys s 2018,8, 637 6 o 39
wi h his ca alys , he o ma ion o me hyl lac a e shows wo dis inc kine ic egimes, whe e mos o
his p oduc is o med in he slow eac ion egime, which esul s om he accumula ion o me hyl
uc oside as a masked o m o glucose. Mo eo e , h ough me hyl uc oside, he kine ics o he
eac ion can be accele a ed in he p esence o small amoun s o wa e as long as hey do no a ec he
ca alys s abili y [34].
Th ough dehyd a ion o glucose o 5-HMF and u he ehyd a ion o his molecule, le ulinic acid
has been ob ained wi h C -modi ied HZSM-5 zeoli e. Fo his eac ion, he kine ics we e s udied and,
applying a simpli ied kine ic model, he calcula ed ac i a ion ene gies o glucose ehyd a ion and
5-HMF ehyd a ion we e 69.1 and 54 kJ mol
−1
, espec i ely. F om he kine ic analysis, i was seen ha
he 5-HMF ehyd a ion cons an was highe han ha o he dehyd a ion o glucose, which implies ha
he la e is he con olling s ep [35].
Finally, u u al can be p oduced om isome iza ion ollowed by he clea age o a C-C bond and
dehyd a ion wi h
β
-zeoli es [
36
]. In addi ion, he sol en used in he ans o ma ion appea s o play a
ole in he ca aly ic pe o mance. Zhang e al. conduc ed he con e sion o glucose o u u al wi h
Sn-
β
zeoli e and ound ha using
γ
- ale olac one/wa e in he eac ion medium enhanced yields o
he p oduc compa ed o wa e , DMSO o γ-bu y olac one [37].
Ca alys s 2018,8, 637 7 o 39
Table 1.
Summa y o he de ails o wo k ound in he li e a u e dealing wi h he ans o ma ion o glucose (glu) o a ious p oduc s h ough hyd oxyme hyl u u al
(5-HMF) ela ed ou es.
Reac ion P oduc O he P oduc s Ca alys Reac ion Condi ions Main Resul s Re e ence
Isome iza ion F uc ose
(F u)
Mannose Decomposi ion
p oduc s LiB
T = 120 ◦C
Cglu =10gL−1in wa e
Cca = 60% w/w
= 15 min
Xglu = 51.8%
Y u = 30.3%
TOF = 0.01 mol u molca −1h−1
[23]
Isome iza ion F uc ose Mannose Cu(NO3)2and o he
Cu-con aining ca alys s.
T = 110 ◦C
Cglu = 1% w/win wa e
Cca = 60% w/w
pH = 5.3
= 90 min
Xglu = 18%
Y u = 16%
TOF = 0.45 mol u molca −1h−1
[24]
Isome iza ion -e he i ica ion Me hyl uc oside
(MF) F uc ose
Zeoli es H-USY, H-Y, H-β
(Lewis acidi y)
Dowex 50WX8-100 (B øns ed
acidi y) Si/Al a io = 30
T = 120 ◦C
Cglu = 3.13% w/win MeOH
Cca = 60%
= 60 min
Xglu = 83%
YMF = 72%
TOF = 84.24 molMF gca −1h−1
[25]
Isome iza ion-e he i ica ion Me hyl uc oside F uc ose
Kaolin: S em-2008 and o he
kaolin samples
SBET = 16 m2g−1
T = 120 ◦C
Cglu = 3% w/win MeOH
Cca = 60% w/w
= 900 min
Xglu = 93%
YMF = 52%
TOF = 1.15 molMF gca −1h−1
[26]
Isome iza ion Dehyd a ion 5-HMF F uc ose
Sul anilinic acid, aniline, PTSA,
sulphamic acid
T = 160 ◦C
Cglu = 5% mol in H2O/DMSO/MIBK
Cca = 0.01 M
= 30 min
Xglu = 90%
YHMF = 44%
TOF = 4.4 molHMF molca −1h−1
[27]
Isome iza ion Dehyd a ion 5-HMF F uc ose SnPO ( om Sn3(PO4)4)
SBET = 120.8 m2g−1
T = 120 ◦C
Cglu = 20% w/w% in [Emim][B ]
Cca = 10% w/w
= 180 min
Xglu = 94.1%
YHMF = 58.3%
TOF = 2.16 molHMF gca −1h−1
[28]
Isome iza ion Dehyd a ion 5-HMF F uc ose
Chi osan nanopa icles doped
wi h C (III) and C (VI) ions
aided by H2SO4
SBET = 30.4 m2g−1
CC (III) = 3%
T = 180 ◦C
Cglu = 3.13% w/win wa e /DMSO
Cca = 5% w/w
= 180 min
Xglu = 92.3%
YHMF = 64.7%
TOF = 0.75 molHMF gca −1h−1
[29]
Isome iza ion Dehyd a ion 5-HMF F uc ose SnPCP@MnO2–PDA
SBET = 240.6 m2g−1
T = 150 ◦C
Cglu = 4% w/win DMSO
Cca = 1% w/w
= 300 min
Xglu = 92.2%
YHMF = 55.8%
TOF = 2.47 molHMF gca −1h−1
[38]
Isome iza ion Dehyd a ions
Hyd ogenolysis 2,5-DMF
F uc ose, 5-HMF,
5-me hyl u u al,
2,5-bis(hyd oxyme hyl)- u an;
5-me hyl- u anme hanol
4.8Pd/UiO-66@SGO (Pd on a
Z -based MOF deposi ed on
sul ona ed g aphene oxide)
SBET = 715 m2g−1
CPd = 4.8%
T = 160 ◦C
PH2 = 1 MPa
Cglu = 0.025 M in THF
Cca = 0.5% w/w
= 180 min
Xglu = 87.3%
YDMF = 45.3%
TOF = 0.76 molDMF gca −1h−1
[30]
Ca alys s 2018,8, 637 8 o 39
Table 1. Con .
Reac ion P oduc O he P oduc s Ca alys Reac ion Condi ions Main Resul s Re e ence
Isome iza ion Re o-aldol
agmen a ion Dehyd a ion
Ace aliza ion Isome iza ion
Me hyl lac a e
(MeLac)
F uc ose, glyce aldehyde,
dihyd oxyace one,
py u aldehyde, among many
o he s epo ed
ZnCl2and o he Zn(II) sal s
T = 200 ◦C
Cglu = 0.4% w/win E OH and wa e
Cca = 0.004 M
= 180 min
YMeLac = 47.7%
TOF = 0.88 molMeLac
molca −1h−1
[33]
Isome iza ion Re o-aldol
agmen a ion Dehyd a ion
Ace aliza ion Isome iza ion
Me hyl lac a e
(MeLac)
F uc ose, uc o u anosides,
uc opy anosides
Sn-Be a zeoli e
SBET = 722 m2g−1
CSn = 0.977%
T = 160 ◦C
Cglu = 0.132 M in E OH
Cca = 1% w/w
= 720 min
YMeLac = 43%
TOF = 0.47 molMeLac gca −1h−1
[34]
Dehyd a ion Rehyd a ion Le ulinic acid
(Le Ac) 5-HMF, o mic acid, humins
C -HZSM-5
SBET = 308.9 m2g−1
CC = 7.25%
T = 180 ◦C
Cglu = 2% w/win wa e
Cca = 0.75% w/w
= 180 min
Xglu = 100%
YLe Ac = 64.4%
TOF = 3.18 molLe Ac gca −1h−1
[35]
Isome iza ion, C-C bond
clea age Dehyd a ion Fu u al F uc ose, 2,5-HMF,
lac ic acid, a abinose H-βzeoli e
T = 150 ◦C
Cglu = 5% w/win γ- ale olac one and
wa e
Cca = 1% w/w
PN2 = 2 MPa
= 60 min
Xglu = 99.9%
YFu = 56.5%
TOF = 15.68 molFu gca −1h−1
[36]
Isome iza ion Dehyd a ion Fu u al 5-HMF
Sn, Fe and Z -βzeoli e
SBET = 539.9 m2g−1
CSn = 7.1 µmol g−1
T = 180 ◦C
Cglu = 0.6% w/win γ- ale olac one and
wa e
Cca = 2.4% w/w
= 33 min
Xglu = 100%
YFu = 69.2%
TOF = 1.75 molFu gca −1h−1
[37]
No e: SBET s ands o speci ic su ace measu ed by BET iso he m; ca s ands o ca alys .
Ca alys s 2018,8, 637 9 o 39
2.2. O he Rou es om Glucose o Value-Added P oduc s
Some p oduc s ob ained a oiding 5-HMF ou es a e included in Figu e 3, whose de ails o he
co esponding syn hesis eac ions a e compiled in Table 2.
Aldohexose mannose can be p oduced om he epime iza ion and hyd ogena ion o glucose,
o which a con inuous low sys em has been implemen ed. The au ho s compa ed he pe o mance o
using a dual ca aly ic sys em consis ing o Cs suppo ed on he e opolyacid o e ca bon (Cs-HPA/C)
and Ru suppo ed on ca bon simul aneously o in se ies, he la e op ion being op imal using
i s he Cs-HPA/C o epime iza ion and secondly a mix u e o bo h ca alys s o also pe o m
hyd ogena ion [39].
Lac ic acid has been ob ained om glucose wi h a bi unc ional Al(III)-Sn(II) ca alys ia a se ies o
andem s eps ha include he isome iza ion o uc ose and hen he e o-aldol agmen a ion in o
wo C3 in e media es ha e en ually dehyd a e and isome ize o lac ic acid. Al(III) is accoun able
o isome iza ion ia a 1,2-hyd ide shi mechanism and con e sion o ioses o lac ic acid; o i s
pa , Sn(II) is liable o he agmen a ion o uc ose in o wo C3 in e media es (dihyd oxyace one and
py u aldehyde) [40].
By hyd ogena ion and dehyd a ion, D-isoso bide can be ob ained using a one-po syn hesis
employing a combined he e ogeneous ca alys o Ru suppo ed on a Dowex-H gel- ype exchange esin.
The yields epo ed we e o up o 85% a e 48 h o eac ion in a p essu ized au ocla e [41].
The p oduc ion o glycaldehyde,
α
-hyd oxy-
γ
- ale olac one and D- e oses (D-(-)e y h ose and
D-(+)-e y h ulose) was pu sued in a di e en s udy, whe e hey analyzed he p o ile o he p oduc s
om he con e sion o glucose wi h di e en ca alys s. Ammonium ungs a e ga e he bes con e sion
o 94.4%, ob aining mos ly glycaldehyde as p oduc , wi h a yield o 52.5% [42].
The oxida ion o gluconic acid wi h excep ional high yields o 98% was pe o med
pho oca aly ically, achie ing p ac ically o al con e sion o glucose. Fo his, Au nanopa icles we e
suppo ed on TiO
2
and subjec ed o eac ion unde i adia ion be ween 420 and 780 nm a e y
mild empe a u e condi ions [
43
]. In ano he s udy, no only gluconic acid bu also o ma es we e
ob ained coupling dehyd ogena ion and (NH
4
)
2
CO
3
hyd ogena ion by ans e leading o so bi ol as
a side-p oduc . In his case, Pd and P suppo ed on ac i a ed ca bona e we e used simul aneously
o 24 h ob aining con e sions o 72.6% and yields o gluconic acid o almos 60% and o ma es in
abou 32% [
44
]. Finally, p oduc ion o gluconic acid has also been a ained by pho ooxida ion o
uc ose, al hough xyli ol has addi ionally been epo ed as a u he decomposi ion p oduc . Fo his
eac ion, TiO
2
ca alys s we e syn hesized, o which he one p epa ed by he sol-gel me hod using
polye hylene glycol as su ac an was mos a o able o ob ain gluconic acid, whe eas using ul asound
and ce yl ime hylammonium b omide p o ed mo e e icien o p oduce xyli ol [45].
Succinic acid can also be ob ained as an oxida ion p oduc s a ing om glucose as subs a e.
In his way, Ga cía e al. ha e pe o med he oxida ion and u he clea age using ni ogen-doped
g aphene as ca alys eaching yields as high as 68%, wi h he N loading playing a c ucial ole in he
pe o mance, eaching an op imum a 3.8% [
46
]. When ca bon nano ubes doped wi h Fe and V we e
used, he majo p oduc o he oxida ion eac ion was oxalic acid (yield o 46%), wi h some succinic
acid being obse ed as well, al hough o a much lowe ex en (only 7.8%) [47].
Las , a inylized p oduc ha can be used as a monome o in e es ing ma e ials has also been
p epa ed om uc ose, as obse ed in Figu e 3. The p ocedu e o p epa e i consis ed o i s p o ec ing
hyd oxyl moie ies by ace aliza ion wi h ace one and u he inyla ion. Fo he o me s ep an acidic
ca alys like sul u ic acid was pu o use, while o he la e , calcium ca bide aided by basic compounds
like KF and KOH. This p ocess also depended on he polyme iza ion o monome s p oduced om
di e en suga s [48].
Ca alys s 2018,8, 637 16 o 39
Table 3. Con .
Reac ion P oduc O he P oduc s Ca alys Reac ion Condi ions Main Resul s Re e ence
Dehyd a ion
Hyd olysis
Hyd ogena ion
1-hyd oxy-2,5-hexanedione
5-HMF
2,5-bis-(hyd oxyme hyl)
u an
1s s ep o 5-HMF: HCl
2nd s ep o 1-hyd oxy-2,5-hexanedione:
Cp*I III hal -sandwich complexes wi h
bipy idine ligands
CI = 3.64 mgL−1
1s s ep o 5-HMF: T = 130
◦
C; C
u
= 0.5 M
in IPA/wa e
CHCl = 0.05 M
2nd s ep o 1-hyd oxy-2,5-hexanedione
T = 130 ◦C
C5-HMF = 0.517 M in aq. o ma e bu e
solu ion (pH = 2.5) Cca = 0.517 M
= 180 + 120 min
X u = 71.9%
YHDone = 99%
TOF = 0.19 molHDone
molca −1h−1
[55]
Dehyd a ion,
e o-condensa ion
Isome iza ion
Es e i ica ion
Me hyl lac a e
5-HMF
glyce aldehyde DHA
uc osides
Hie a chical Sn-βZeoli e
SBET = 719 m2g−1
Si/Al a io = 12.5
CSn = 3.7%
T = 160 ◦C; C uc ose = 0.15 M in me hanol
Cca = 0.5% w/w
PN2 = 1 MPa
= 1200 min
X u = 100%
YML = 86%
TOF = 1.29 molMeLac gca −1h−1
[57]
Re o-aldol
agmen a ion
Dehyd a ion
Ace aliza ion
isome iza ion
Me hyl lac a e Glyce aldehyde, dihyd oxyace one,
py u aldehyde, among many o he s epo ed ZnCl2
T = 200 ◦C; C u = 0.4% w/w
in E OH and wa e
Cca = 0.004 M
= 180 min
YML = 52%
TOF = 0.96 molMeLac molca −1h−1
[33]
Re o-aldol
agmen a ion
Isome iza ion
Es e i ica ion
Me hyl lac a e Me hyl le ulina e and uc osides
InCl3.4H2O/Bu2SnCl2and o he In–Sn
ca aly ic sys ems
In/Sn a io = 5
T = 160 ◦C
C u = 0.125 M in me hanol
Cca = 0.5% w/w
PN2 = 0.5 MPa
= 600 min
X u = 98%
YMeLac = 72%
TOF = 1.22 molMeLac molca −1h−1
[58]
Mechanism is
no discussed Me hyl lac a e No epo ed Sn-βzeoli es
SBET = 422 m2g−1
T = 160 ◦C
C uc ose = 2.5% w/win me hanol; Cca =
1.6% w/w
PN2 = 0.5 MPa
= 600 min
YMeLac = 47%
TOF = 0.39 molMeLac gca −1h−1
[59]
Hyd o he mal
Decomposi ion Le ulinic acid 5-HMF [P SO3HMIm]
[Cl] and o he ionic liquids
T = 180 ◦C; C u = 2% w/w
in wa e
Cca = 40% w/w
= 180 min
X u = 100%
YLe Ac = 79%
TOF = 0.31 molLe Ac molca −1h−1
[60]
Dehyd a ion
E he i ica ion
Ace aliza ion
Hyd a ion
E hyl le ulina e 5-HMF, u u al, e hoxy u u al
Ti
0.75
TPA and o he i anium exchanged
he e opoly TPA
T = 120 ◦C
C u = 0.25 M
in E OH; Cca = 2.25% w/w
= 360 min
X u = 100%
YE Le = 63%
TOF = 1.17 molE Le gca −1h−1
[61]
Dime iza ion
Oxida ion
Dica boxylic acid monome :
5,5’-[oxybis
(me hylene)]bis
[2- u anca boxylic acid]
5,5’-[oxybis
(me hylene)]bis
[2- u aldehyde] (OBFA)
Dime iza ion: Dowex 50 W X8
Oxida ion:
5% P /C
Dime iza ion: T = 110 ◦C
C u = 45% w/w
in DMSO
Cca = 10% w/w.
Oxida ion:
T = 23 ◦COBFA = 0.2 M in aq. NaOH (1.5
M) Cca = 2% w/w; PO2 = 1 a m
= 1440 + 2880 min
X u = 100%
Ymonome = 75%
[62]
C-C bond clea age
and dehyd a ion Fu u al
Glucose
2,5-HMF
Lac ic acid A abinose
H-βzeoli e
Si/Al a io = 25
Cacid = 0.366 mmol gca −1
T = 150 ◦C
C u = 5% w/win GBL/wa e
Cca = 1% w/w
PN2 = 20 ba
= 60 min
X u = 100%
YFu = 64%
TOF = 1.78 molFu gca −1h−1
[36]

Ca alys s 2018,8, 637 17 o 39
Table 3. Con .
Reac ion P oduc O he P oduc s Ca alys Reac ion Condi ions Main Resul s Re e ence
Dehyd a ion Fu u al 5-HMF
Sn, Fe and Z -Be a zeoli e
SBET = 539.9 m2g−1
CSn = 7.1 µmol g−1
T = 170 ◦C
C u = 0.6% w/w
in GBL/wa e
Cca = 2.4% w/w
= 30 min
X u = 100%
YFu = 69%
TOF = 1.92 molFu gca −1h−1
[37]
Dehyd a ion
E he i ica ion
E hoxyme hyl
u u al
5-HMF
E hyl le ulina e
A -SO3H-SBA-15 and o he
mesopo ous silica
SBET = 712 m2g−1
T = 116 ◦C, C u = 0.2 M
in e hanol/DMSO (91.7:8.3 / )
Cca = 0.027 M
= 240 min
X u = 100%
YEMFu = 64%
TOF = 1.19 molEMFu gca −1h−1
[63]
Dehyd a ion
E he i ica ion
E hoxyme hyl
u u al HMF, lac ic acid
lignin-de i ed sulpha ed ca bon
SBET = 26 m2g−1
CS= 36 M
T = 150 ◦C
C u = 2% w/w
in e hanol Cca = 0.5% w/w
PN2 = 20 ba
= 180 min
X u = 100%
YEMFu = 64%
TOF = 3.03 molEMFu gca −1h−1
[64]
Dehyd a ions
Hyd ogena ions
Hyd ogenolysis
2,5-DMF
F uc ose, 5-HMF, 5-MFA (5-me hyl u u al),
2,5-BHMF (2,5-bis(hyd oxyme hyl)- u an;
5-MFM (5-me hyl- u anme hanol)
4.8Pd/UiO-66@SGO (Pd on a Z -based
me alo ganic amewo k deposi ed on
sul ona ed g aphene oxide)
SBET = 715 m2g−1
CPd = 4.8%
T = 160 ◦C
PH2 = 1 MPa
C u = 0.5% mol
in THF
Cca = 0.5% w/w
= 180 min
X u = 92%
YDMF = 71%
TOF = 1.18 molDMF gca −1h−1
[30]
Dehyd a ion
Aldol condensa ion
wi h me hylisobu yl
ke one
(E)-1-(5-(hyd oxyme hyl)
u an-2-yl)-5-me hylhex
-1-en-3-one
5-HMF Dehyd a ion: KB , H2SO4
Aldol condensa ion: NaOH
Dehyd a ion:
T = 150 ◦C
C u = 0.055 M
in dioxane CKB = 0.0375 M
CH2SO4 = 0.125 M
Aldol condensa ion:
T = 55 ◦C
CMIBK = 1 M in dioxane
= 1 + 180 min
X u = 100%
Yp oduc = 73%
TOF = 0.36 molp oduc molca −1h−1
[65]
Ca alys s 2018,8, 637 18 o 39
3.2. Addi ional Rou es om F uc ose o Fu he P oduc s
Ve y much like in he case o glucose, compounds o in e es can be syn hesized om uc ose
om eac ion pa hways no depa ing om dehyd a ion, which a e shown in Figu e 4and de ailed in
Table 4.
Suga alcohol manni ol can be p oduced om he hyd ogena ion o uc ose, o which
coppe -suppo ed me allic nanopa icles we e syn hesized, whe e he bes was a Cu ca alys suppo ed
on silica by p ecipi a ion-deposi ion me hod compa ed o p epa a ion by incipien we ness me hod,
which ga e ca alys s wi h lowe ac i i y and selec i i y o manni ol [67].
Oxida ion and clea age wi hin he molecule leading o oxalic and succinic acid ha e also been
epo ed s a ing om uc ose, o which i on suppo ed on ca bon nano ubes has been employed.
Fo mic acid is ob ained as a by-p oduc o he eac ion, also o use in many applica ions. These ca alys s
showed good eusabili y wi hou much loss o e iciency [47].
The wo k by Rodygin e al. epo ed abo e also s udied he p oduc ion o a inylized p oduc as
monome o u he polyme iza ion, as obse ed in Figu e 4 o he case o uc ose as subs a e o he
eac ion. Using he same s eps o he eac ion and ma e ials o he p o ec ion o hyd oxyl moie ies
and inyla ion, a yield o 92% was a ained in he end [48].
Ano he example is he syn hesis o lac ic acid p oduc ion, which is based on consecu i e s eps
o e o-aldol agmen a ion, dehyd a ion and isome iza ion also using bi unc ional Al(III)-Sn(II) as
a ca alys , as desc ibed abo e o glucose alo iza ion. When uc ose was he subs a e, somewha
highe yields o lac ic acid o 90% we e ob ained compa ed o he 81% om glucose [40].
Finally, i is also wo h b ie ly men ioning he syn he ic app oach o he p oduc ion o
3-deoxy-L- uc ose ha Lu and Chang ca ied ou , which consis ed o a se ies o s eps o benzoyla ion,
b omina ion, di hioace aliza ion and inal emo al o he benzoyl and di hiane g oups o ob ain he
desi ed p oduc [68].
Ca alys s 2018, 8, x FOR PEER REVIEW 18 o 40
3.2. Addi ional Rou es om F uc ose o Fu he P oduc s
Ve y much like in he case o glucose, compounds o in e es can be syn hesized om uc ose
om eac ion pa hways no depa ing om dehyd a ion, which a e shown in Figu e 4 and de ailed
in Table 4.
Suga alcohol manni ol can be p oduced om he hyd ogena ion o uc ose, o which coppe -
suppo ed me allic nanopa icles we e syn hesized, whe e he bes was a Cu ca alys suppo ed on
silica by p ecipi a ion-deposi ion me hod compa ed o p epa a ion by incipien we ness me hod,
which ga e ca alys s wi h lowe ac i i y and selec i i y o manni ol [67].
Oxida ion and clea age wi hin he molecule leading o oxalic and succinic acid ha e also been
epo ed s a ing om uc ose, o which i on suppo ed on ca bon nano ubes has been employed.
Fo mic acid is ob ained as a by-p oduc o he eac ion, also o use in many applica ions. These
ca alys s showed good eusabili y wi hou much loss o e iciency [47].
The wo k by Rodygin e al. epo ed abo e also s udied he p oduc ion o a inylized p oduc
as monome o u he polyme iza ion, as obse ed in Figu e 4 o he case o uc ose as subs a e
o he eac ion. Using he same s eps o he eac ion and ma e ials o he p o ec ion o hyd oxyl
moie ies and inyla ion, a yield o 92% was a ained in he end [48].
Ano he example is he syn hesis o lac ic acid p oduc ion, which is based on consecu i e s eps
o e o-aldol agmen a ion, dehyd a ion and isome iza ion also using bi unc ional Al(III)-Sn(II) as
a ca alys , as desc ibed abo e o glucose alo iza ion. When uc ose was he subs a e, somewha
highe yields o lac ic acid o 90% we e ob ained compa ed o he 81% om glucose [40].
Finally, i is also wo h b ie ly men ioning he syn he ic app oach o he p oduc ion o 3-deoxy-
L- uc ose ha Lu and Chang ca ied ou , which consis ed o a se ies o s eps o benzoyla ion,
b omina ion, di hioace aliza ion and inal emo al o he benzoyl and di hiane g oups o ob ain he
desi ed p oduc [68].
Figu e 4. Reac ion pa hways using uc ose as building block a oiding ou es s a ing om 5-HMF.
Figu e 4. Reac ion pa hways using uc ose as building block a oiding ou es s a ing om 5-HMF.
Ca alys s 2018,8, 637 19 o 39
Table 4. In o ma ion in s udies on he con e sion o uc ose o a ious p oduc s ia al e na i e ou es.
Reac ion P oduc O he P oduc s Ca alys Reac ion Condi ions Main Resul s Re e ence
Hyd ogena ion Manni ol So bi ol
Glucose
Cu/SiO2-PD and o he
coppe -suppo ed me allic nanopa icles
SBET = 225 m2g−1
CCu = 11.3%
T = 200 ◦C
PH2 = 40 ba
C u = 0.055 M
in E OH/wa e
Cca = 0.5% w/w
= 360 min
X u = 100%
YMan = 78%
TOF = 1.43 molmanni ol gca −1h−1
[67]
Oxida ion and clea age
Oxalic and Succinic acid
F uc ose,
2- o myl-5- u anca boxylic
acid, o mic acid
Fe@CNT
SBET = 78 m2g−1
T = 140 ◦C
Cglu = 0.05 M in wa e
Cca = 2.5 g L−1
PO2 = 20 ba
=12h
Xglu = 99%
YOxAc = 46.8%
YSucAc = 21%
TOF = 0.003 molOxAc gca −1h−1
[47]
Ace aliza ion ( o
p o ec ion) Vinyla ion
Vinylized
monome Ace alized in e media e Ace aliza ion: H2SO4
Vinyla ion: CaC2(KF, KOH)
T = 130 ◦C
C u = 0.33 M
in DMSO/wa e
CCaC2 = 1.2 M
CKOH/KF = 0.22 M
= 180 min
Ymonome = 92%
TOF = 1.29 molmonome
molca −1h−1
[48]
Re o-aldol
agmen a ion
Dehyd a ion
Isome iza ion
Lac ic acid
Glyce aldehyde
Dihyd oxyace one
Py u aldehyde
Bi unc ional Al(III)-Sn(II) ca alys s
T = 180 ◦C
C u = 0.5% w/w
in wa e
pH = 2.8
Cca = 0.005 M
Al(III)/Sn(II) a io = 1/1
= 120 min
X u = 100%
YLA = 90%
TOF = 2.50 molLacAc molca −1h−1
[40]
Ca alys s 2018,8, 637 20 o 39
4. Ca aly ic Rou es om Xylose
Xylose, oge he wi h mannose, a pen ose and a hexose, a e he main componen s o hemicelluloses
o igina ing om wood. Fo example, in so woods, he xylan sha e anges be ween 3 and 8% w/wd y
solid (DS), while mannans a e p esen in a highe pe cen age— om 10 up o 13% w/wDS, wi h an
o e all con en in hemicelluloses om 25 o 35% w/wDS [19]. Ha dwoods a e iche in hemicellulose
ac ion (24–40%), while g asses and lea es can con ain as much as 85% w/wDS o hemicelluloses,
wi h xylan- and xylose- ela ed polyme s he main componen o he hemicellulosic ac ion in his
case [
69
]. Hemicellulose is o amo phous na u e, has a ela i ely low molecula weigh (app ox. 15 KDa)
and i s composi ion depends highly on he sou ce, which can be e y a ied. They can be polyme s wi h
β
-1,4-linked backbones wi h an equa o ial con igu a ion a C
1
and C
4
, which can be xylans, mannans
and glucomannans, xyloglucans, and
β
-1,3;1,4-glucans, bu also axially-di ec ed
β
-1,4-galac ans and
a abinogalac ans wi h a
β
-1,3-linked galac ose backbone [
69
]. A e being ob ained and pu i ied om
hemicelluloses, xylose is a pla o m chemical ha can be dehyd a ed o u u al, which is in u n an
in e media e o a ple ho a o chemicals, whose exploi a ion can be conduc ed using se e al ca alys s,
as compiled in Tables 5and 6. This pen ose can also be ans o med di ec ly o le ulinic acid, u u al
alcohol and mo e chemicals. All hese pa hways a e summa ized in Figu e 5.
Ca alys s 2018, 8, x FOR PEER REVIEW 20 o 40
4. Ca aly ic Rou es om Xylose
Xylose, oge he wi h mannose, a pen ose and a hexose, a e he main componen s o
hemicelluloses o igina ing om wood. Fo example, in so woods, he xylan sha e anges be ween 3
and 8% w/w d y solid (DS), while mannans a e p esen in a highe pe cen age— om 10 up o 13%
w/w DS, wi h an o e all con en in hemicelluloses om 25 o 35% w/w DS [19]. Ha dwoods a e iche
in hemicellulose ac ion (24–40%), while g asses and lea es can con ain as much as 85% w/w DS o
hemicelluloses, wi h xylan- and xylose- ela ed polyme s he main componen o he hemicellulosic
ac ion in his case [69]. Hemicellulose is o amo phous na u e, has a ela i ely low molecula weigh
(app ox. 15 KDa) and i s composi ion depends highly on he sou ce, which can be e y a ied. They
can be polyme s wi h β-1,4-linked backbones wi h an equa o ial con igu a ion a C1 and C4, which
can be xylans, mannans and glucomannans, xyloglucans, and β-1,3;1,4-glucans, bu also axially-
di ec ed β-1,4-galac ans and a abinogalac ans wi h a β-1,3-linked galac ose backbone [69]. A e
being ob ained and pu i ied om hemicelluloses, xylose is a pla o m chemical ha can be
dehyd a ed o u u al, which is in u n an in e media e o a ple ho a o chemicals, whose exploi a ion
can be conduc ed using se e al ca alys s, as compiled in Tables 5 and 6. This pen ose can also be
ans o med di ec ly o le ulinic acid, u u al alcohol and mo e chemicals. All hese pa hways a e
summa ized in Figu e 5.
Figu e 5. Reac ion ou es om xylose ia u u al and di ec ou es om he pen ose.
Xylose
Dehyd a a ion
Xylulose
Fu u al
O
OH
OH
OH
OH
O
O
O
OH
OH
OH OH
Anhyd oxylose
O
OH
OH
O
Dehyd a a ion
+ educ ion
Fu u yl
alcohol
O
OH
Hyd ogena ion
Re o aldol
condensa ion
OH OH
OH OH
OH
Xyli ol
OH O
CH
3
OH
Es e i ica ion
Lac ic acid
OH
O
CH
3
OCH
3
Me hyl lac a e
Oxida ion
OH OH
OH OH
OH O
Xylonic acid
Oxida ion
OH OH
OH OH
OH O
O
Xyla ic acid
Dehyd a a ion
Dehyd a a ion
Reduc ion Hyd ogena ion
Ring opening
CH
3
OH
O
1-hyd oxyl-2-pen anone
Hyd ogena ion
CH
3
OH
OH
1,2-pen anediol
Hyd ogena ion
Ring opening
O
OH
OH O
CH
3
ans-2,5-dihyd oxy-3-pen enoic
acid me hyl es e (DPM)
T ans e
hyd ogena ion
γ- ale olac one
OO
CH
3
Hyd a ion
Ring opening
CH
3
OH
O
O
Le ulinic acid
T ans e
hyd ogena ion
CH
3
O
OH
O
R
1
4-hyd oxypen anoa es
Figu e 5. Reac ion ou es om xylose ia u u al and di ec ou es om he pen ose.
Ca alys s 2018,8, 637 21 o 39
Table 5.
In o ma ion con ained in pape s on he dehyd a ion o xylose o u u al in liquid-liquid sys ems (ca alys dissol ed in one liquid phase) o liquid
(homogeneous) sys ems.
Reac ion P oduc O he P oduc s Ca alys Reac ion Condi ions Main Resul s Re e ence
Dehyd a a ion
Resini ica ion o
sel -polyme iza ion
Fu u al poly u u al Te eph halic
acid (TPA)
T = 190 ◦C
Cxyl = 8.9% w/win H2O V oluene/VH2O = 2
Cca = 0.5% w/
= 180 min
Xxyl = 92%
YFu = 72%
100% s able o 5 cycles
TOF = 0.05 molFu gca −1h−1
[70]
Dehyd a ion
Polyme iza ion Fu u al
F om xylan:
Humins
Monosaccha ides
F om xylose:
No indica ed
(Low conc.)
Ionic liquid
[Choline-SO4H][CF3SO3]
T = 120 ◦C
Cxyl = 40% in 1,4-dioxane wi h 2% H2O
(Cxylan iden ical)
Cca = 2% w/
F om xylan = 360 min
Xxyl = 64%
YFu = 62.4%
TOF = 2.3·10−3molFu gca −1h−1
F om xylose = 600 min
Xxyl = 99.5%
YFu = 91.5%
TOF = 3.36
·
10
−3
mol
Fu
g
ca −1
h
−1
[71]
Dehyd a ion Fu u al F om xylose:
xylulose C PO4
T = 160 ◦C
Cxyl = 10% w/ in H2O
V oluene/VH2O = 3
CNaCl = 35% w/wH2O
Cca = 1.5% w/wH2O
= 60 min
Xxyl = 98%
YFu = 88%
TOF = 0.094
molFu gca −1h−1
[72]
Dehyd a ion
Polyme iza ion
Resini ica ion
Fu u al Humins
Fu u al polyme s HCl
T = 222 ◦C
Cxyl = 10% w/ in H2O VGVL/VH2O = 4
Cca = 5 mM
Tubula eac o
Liquid sys em
=96s
Xxyl = 93%
YFu = 82%
TOF = 8.57
molFu mmolca −1h−1
Maple:
= 30 min
Xxyl = 93%
YFu = 82%
TOF = 0.084
molFu mmolca −1h−1
[73]

Ca alys s 2018,8, 637 22 o 39
Table 6. In o ma ion con ained in s udies on he dehyd a ion o xylose o u u al in solid-liquid sys ems (solid ca alys s).
Reac ion P oduc O he P oduc s Ca alys Reac ion Condi ions Main Resul s Re e ence
Dehyd a a ion
Isome iza ion Fu u al xylulose
C -MOF wi h Sn phospha e nanopa icles
SBET = 1000–2820 m2g−1
Cacid = 0.41–0.62 mmol gca −1
T = 150 ◦C
Cxyl = 10% w/win H2O V oluene/VH2O = 7/3
Cca = 3% w/ H2O
CNaCl = 70 pp
= 180 min
Xxyl = 97%
YFu = 95%
100% s able up o 10 cycles
TOF = 0.022 molFu gca −1h−1
[74]
Dehyd a a ion
Polyme iza ion Fu u al humins
Ambe ly s 70
M-20
ZSM-5-30
Cacid = 0.42–4.15 mmol gca −1
T = 150 ◦C
Cxyl = 10% w/win H2O
PCO2 = 20 MPa
QCO2 = 0.94 g min−1
Cca = 10% w/ H2O
Ambe lys 70
=16h
Xxyl = 91.4%
YFu = 50.5%
TOF = 1.24 ×10−3molFu gca −1h−1
[75]
Dehyd a ion
Polyme iza ion Fu u al Humins
SO3H-KIT-6
SBET = 265 m2g−1
Cacid = 0.69–1.53 mmol gca −1
T = 170 ◦C
Cxyl = 4% w/ in H2O
Cca = 25% w/wH2O
= 120 min
Xxyl = 97.5%
YFu = 94.7%
TOF = 1.99 ×10−4molFu gca −1h−1
[76]
Dehyd a ion
Polyme iza ion
Resini ica ion
Fu u al Humins
Anhyd oxylose
NbTiO-MNL
SBET = 145 m2g−1
Cacid = 0.69–1.53 mmol gca −1
T = 130 ◦C
Cxyl = 20 mM
VGVL/VH2O = 9
Wca = 280 mg
esidence = 106 s
Xxyl = 98%
YFu = 29%
TOF = 7.26 molFu gca −1h−1
[77]
Dehyd a ion
Polyme iza ion Fu u al Humins
FDU and SBA mesopo ous ca alys s
SBET = 500–900 m2g−1
Cacid = 0.07–0.53 mmol gca −1
T = 160 ◦C
PN2 = 2 MPa
Cxyl = 5% w/ in H2O
V oluene/VH2O = 2/1
Cca = 2.5% w/ H2O
FDU-5-7.5E-SO3H
= 240 min
Xxyl = 96.81%
YFu = 78.55%
TOF = 5.13 ×10−3molFu gca −1h−1
[78]
Dehyd a ion Fu u al No indica ed
Sul ona ed g aphi ic ca bon ni ides
SBET = 10–35 m2g−1
Cacid = 5.47 mmol gca −1
T = 100 ◦C
Wxyl = 30 mg in H2O
Wca = 25 mg
= 30 min
Xxyl = 100%
YFu = 96%
TOF = 5.36 ×10−3gFu gca −1h−1
[52]
Dehyd a ion Fu u al No indica ed
Sul ona ed ac i e ca bons (CA)
SBET = 620–750 m2g−1
Cacid = 0.43–0.90 mmol gca −1
T = 180 ◦C
Cxyl = 1.4% w/ in H2O
Cca = 0.14% w/wH2O
KOH-ac i a ed CA
= 180 min
Xxyl = 95%
YFu = 60%
TOF = 0.0152 molFu gca −1h−1
[79]
Dehyd a ion
Es e i ica ion
Reduc ion
Ring opening
T ansloca ion
Hyd ogena ion
Fu u al
Xylose e he s
Fu u yl alcohol
Lac ones
GVL
Le ulinic acid
Hyd oxy-pen anoa es
Z -USY zeoli es wi h se e al Al/Z a ios
SBET = 308–418 m2g−1
Cacid = 0.137–0.650 mmol gca −1
T = 170 ◦C
Cxyl = 1 mol o 50 mol
2-p opanol
Cca = 1% w/
Pa en H-USY
= 180 min
Xxyl = 100%
YFu = 40%
TOF = 3.48 ×10−3molFu gca −1h−1
= 60 min
Xxyl = 80%
YXyle he s = 70%
TOF = 8.21 ×10−3molXyle he s gca −1h−1
[80]
Ca alys s 2018,8, 637 23 o 39
4.1. Dehyd a ion o Xylose o Fu u al
Fu u al is a key building block o p oduce se e al uels and chemicals (lac ones, ce ones, le ulinic
acid, e c.) ha can de i e om xylose making use o bo h homogeneous and he e ogeneous ca alysis.
To o e come he sho comings o homogeneous sys ems using classic mine al B øns ed acids, he use
o he e ogeneous ca aly ic sys ems (liquid-liquid and solid-liquid) has been deeply analyzed in ecen
decades [16] wi h he po en ial capabili y o mo e easily ecycling he ca alys s.
When using liquid-liquid sys ems, u u al is usually eco e ed in he o ganic sol en , while he
ca alys and he emaining xylose emains in he aqueous phase. Recen ly, e eph halic acid has been
employed ecen ly as a soluble ca alys o he p oduc ion o u u al wi h high yields (up o 72–74%),
using aqueous solu ions o he xylose subs a e suspended in up o wo olumes o oluene. In his way,
his acid ha can be ob ained om inexpensi e PET, can be used up o i e imes wi h no educ ion in
ac i i y (e en a sligh ac i a ion up o 5% is obse ed), as u u al is ex ac ed mos ly in he oluene
phase du ing each cycle, adding mo e xylose o he aqueous phase o he nex ca aly ic cycle [
70
].
In he wo k o Hui e al., se e al new SO
4
H- unc ionalized ionic liquids (SFILs) we e syn he ized
and applied o he p oduc ion o u u al om xylose, showing [Ch-SO
4
H][CF
3
SO
3
] wi h he highes
ac i i y. This liquid ca alys can be easily sepa a ed om he aqueous phase simply by decan a ion,
as i shows no miscibili y wi h he xylose-con aining phase. Fo ecycling, he SFIL can unde go
ea men a 80
◦
C unde acuum o emo e all ola ile impu i ies and be subsequen ly used o he
nex ca aly ic cycle. Up o i e cycles we e pe o med o es he s abili y o he ca alys , esul ing in a
sligh 5% dec ease bo h in xylose con e sion ( educed om 98 o 92%) and u u al yield ( ha d opped
om 95 o 90%). The ca alys was also di ec ly applied o xylan, wi h a educ ion om 80 o 60% in he
yield o u u al [
71
]. A hi d liquid-liquid ca aly ic sys em was designed o con e ei he xylose o
whea s aw by he ac ion o he B øns ed and Lewis acidi y o ch omium phospha e. Among se e al
sal s es ed, his ca alys was able o yield 88% u u al wi h o al con e sion o xylose, a 160
◦
C a e
60 min, using oluene as he bes hyd ophobic sol en ou o hose es ed. Again, wo se esul s a e
achie ed i biomass (whea s aw) is di ec ly used: a 180
◦
C du ing 90 min, he yield o u u al was
67% and o 5-HMF, 32%. Unlike wi h he p e ious ca alys s, his one p o ed o be uns able: he ac i i y
owa ds xylose dec eased 10% du ing 4 cycles, bu he yield o u u al sha ply dec eased om 90 o
50%. A possible eason is he p ecipi a ion o C PO4du ing each eac ion cycle [72].
Fu he wo k in liquid sys ems a high empe a u es is p esen ed by Sene e al., whe e he
p oduc ion o u u al om concen a ed aqueous solu ions o xylose is app oached. These au ho s
used he classic ca alys HCl, al hough using
γ
- ale olac one as he main sol en in a mix u e wi h
wa e (80:20 / ). In his case, empe a u es used anged om 200 o 220
◦
C, whe e bo h sol en s
show miscibili y and he concen a ion o he ca alys and he subs a es we e 1–10 mM HCl, 2–10%
w/wxylose in a ubula eac o . In he bes condi ions, yields up o 85% in u u al and quan i a i e
con e sions o xylose a e achie ed in only 90–100 s esidence ime. Resul s we e used o i a powe -law
kine ic model comp ised o he dehyd a ion eac ion xylose o u u al and side eac ions om bo h
chemicals o polyme s, which a e ega ded as by-p oduc s. The app oach is ex ended o a biomass ich
in xylan (maple wood), eaching e en be e esul s using acid hyd olysa es om his sou ce ea ed
a 222
◦
C o 135 s (97% xylan oligome s con e sion and 93% u u al yield). The e o e, he selec ion
o he sol en and he empe a u e seem c i ical o each high u u al yields a o al xylose/xylan
con e sion [73].
The use o solid ca alys s is in es iga ed on a egula basis, comp ising ca alys s wi h B øns ed
acidi y as well as o he s wi h dual B øns ed-Lewis acidi y. O he la e ype, Cha e jee e al. ecen ly
de eloped and es ed me al o ganic ames (MOFs) wi h phospha e g oups and Sn and C a oms,
con aining B øns ed and Lewis acidic si es, espec i ely. S a ing wi h MIL-101, a MOF wi h C a oms,
he addi ion o in phospha e nanopa icles s abilized he ca alys (no change in ac i i y in 10 cycles),
while only 70 pp o NaCl se ed o inc ease xylose con e sion o 99% and u u al yield o 95% [
74
].
The p esence o in phospha e nanopa icles esul ed in wide po es (mesopo e s uc u e) and a oided
he o ma ion o coke and humins deposi s. Again, a hyd ophobic phase, oluene, was used o ex ac
Ca alys s 2018,8, 637 24 o 39
u u al as i was ob ained. Sa o e al. ha e jus published wo k on he applica ion o supe c i ical CO
2
o such pu pose. In hei pape , a he mos able sulphonic acid esin (Ambe lys 70) and wo zeoli es
(M-20 and ZSM-5-30) we e es ed, eaching xylose con e sions up o 91% and u u al yields up o
51% [
75
]. The ca alys s es ed we e also e y ac i e o polyme iza ion due o hei e y s ong acidi y.
The mesopo ous silica KIT-6 has ecen ly been unc ionalized wi h sulphonic g oups o ende a
ca alys able o e icien ly con e xylose in o u u al. Se e al ca alys s we e syn he ized by changing
he mola a io be ween 3-me cap op opyl(me hyl)dime hoxysilane (MPMDMS) and e ae hoxysilane
(TEOS). A 170
◦
C, he bes ca alys (mola a io TEOS-MPMDMS 8/2) could ans o m 97.5% o he
pen ose, achie ing yields up o 94.6% u u al. Howe e , while he capaci y o he 0.2-SO
3
H-KIT-6
was main ained o ou cycles, i s selec i i y o u u al d ama ically dec eased om 95 o 65%,
wi h an inc easing impac o coking side eac ions [
76
]. Mo eno-Ma odan e al. c ea ed se e al
mac opo ous-mesopo ous ca alys s based on i ania and niobia- i ania, again wi h acid si es o he
B øns ed and Lewis kind. These monoli hic ca alys s we e p epa ed o and es ed in con inuous
low- h ough sys ems ( ixed bed eac o s), showing high s abili y on-s eam o up o 30 h. Howe e ,
selec i i y o u u al was low (25%), while xylose con e sion was s able and high (95–100%) when
wo king a 130
◦
C in a liquid-liquid-solid ixed bed eac o wi h
γ
- ale olac one and wa e mixed
in a a io 9/1 / [
77
]. To acili a e mass ans e wi hin he po ous s uc u e and low- h ough
p ocessing, ano he s a egy ollowed was o c ea e packed s uc u es by liquid phase syn hesis and
e apo a ion-induced-sel -assembly (EISA) me hods. Hu e al. ob ained by hese means o se e al
mesopo ous sul ona ed ca alys s o he FDU and SBA ype. Thei bes ca alys (FDU-5-7.5E-SO
3
H) was
able o con e almos all xylose (97%), ende ing up o 81% yield o u u al a 160
◦
C. Howe e , again,
on successi e eac ion cycles, he yield o u u al dec eased down o 60% a e he i h cycle [78].
By py olysis o u ea, Ve ma e al. c ea ed se e al sul ona ed g aphi ic ca bon ni ide (Sg-CN)
ca alys s, and es ed hem wi h xylose using se e al sol en s and empe a u es. Thei bes esul was
achie ed using wa e , 100
◦
C, and 30 min: a yield o u u al o 96% was eached. The same au ho s,
using hese ca alys s, ob ained up o 84% yield o benzimidazole de i a i es (an impo an in e media e
o d ug syn hesis) om xylose, 5-HMF om uc ose (96%), and le ulinic acid om glucose (41%) [
52
].
Lin e al. ha e also ollowed his s a egy o using enewable ma e ials o ob ain ca alys by py olysis.
In his case, polyme ic by-p oduc s om he dehyd a ion o xylose and hemicelluloses o u u al we e
py olyzed and ac i a ed by se e al means (non-ac i a ed, KOH, ZnCl
2
wi h u he sul ona ion in all
cases). Ac i a ion wi h he hyd oxide o he sal esul ed in wide po es and a di e en dis ibu ion
o ca boxyl and sul onic g oups on he su ace o he ac i e ca bons. All ca alys s we e ac i e o
he dehyd a ion o xylose o u u al, eaching p oduc yields o 60% and xylose con e sions up o
95%. When used di ec ly on hemicellulose, yields o xylooligosaccha ides (XOS) we e as high as 60%,
while u u al yields could each 40% [79].
Apa om he undesi ed side eac ions o humins and u u al polyme s, some ca alys s can
p omo e no only u u al o ma ion bu also i s con e sion in o u u yl alcohol, le ulinic acid,
lac ones—including
γ
- ale olac one—and hyd oxypen anoa es o name a ew ele an low molecula
weigh compounds. This is he case o he wo k by Lopez-Aguado e al., whe e he syn hesis and
es o se e al zeoly ic ca alys s o he ype Z -USY is epo ed. The a ios o Z o Al a e modi ied,
in luencing he dis ibu ion o p oduc s om xylose using 2-p opanol as a sol en , wi h maximum
yields o u u al o 40%, xylose e he s up o 60% and GVL (
γ
- ale olac one) up o 5%. To inc ease
yields o GVL, u u al was employed as he o iginal eac an , eaching o e 13% yield GVL [80].
4.2. Di ec Reac ions o Xylose o Alcohols, Acids and Polyme s
Though u u al is a key in e media e o se e al p oduc s om xylose, some chemicals can
be eached wi hou he p esence o u u al o , a leas , wi hou i s build-up in he liquid phase,
hus inc easing he yields o such chemicals. Resul s a e displayed in Table 7.
In he case o u u yl alcohol, which is he main monome o u anic esins, he combina ion
o sul onic g oups and P ac i e phases on SBA-15 has allowed he di ec p oduc ion o he alcohol
Ca alys s 2018,8, 637 25 o 39
om xylose wi h selec i i ies anging om 83 o 87%, al hough only a 20% xylose con e sion.
The unmodi ied SBA-P ca alys , howe e , mainly yielded xyli ol (45% selec i i y) in simila condi ions.
The main p oblem encoun e ed by Canhaci and cowo ke s in his case was he ex ensi e leaching
o sul onic g oups du ing s abili y s udies. This in u n esul ed in an inc eased yield o xyli ol and
u u al and a e y high educ ion o u u yl alcohol yield in jus ou cycles [81].
A dual ca alys was c ea ed and applied o also ob ain 1,2-pen anediol, he building block
o p opiconazole (DMI)—a iazole ungicide— om xylose. In his wo k, Wang and cowo ke s
used Ru/C as a hyd ogena ion ca alys and niobium phospha e as an acid ca alys . In he bes
condi ions, 21.27% combined yield o 1,2-p opanediol and 1-hyd oxyl-2-pen anone was eached in
a wa e /GVL/cyclohexane biphasic L-L sys em. Mos impo an ly, he au ho s showed ha a high
Lewis acidi y is c i ical o ob ain a high selec i i y o 1,2-p opanediol in hyd ogena ing condi ions,
educing he selec i i y owa ds xyli ol [82].
Xyli ol is he main p oduc om he hyd ogena ion o u u al and a main ing edien in he ood
indus y, as a swee ening agen . Recen ly, Mo ales and cowo ke s syn hesized highly dispe sed NiO
ca alys s ou o mixed oxide p ecu so s ( om Ce and Ni) wi h educed leaching in aqueous medium.
In all cases, he selec i i y o xyli ol was 50–60%, wi h xylulose, glyce ol and e hylene glycol being he
main by-p oduc s [83]. I s ca aly ic p oduc ion has been deeply e iewed ecen ly [84].
Lac ic acid (LA) is a pla o m chemical usually ob ained ia anae obic o mic oae obic
e men a ion o glucose by Lac obacilli. To inc ease p oduc i i ies, wo king wi h mo e concen a ed
eeds ock, he ca aly ic s a egy could be an al e na i e. Liu e al. ha e ecen ly p o ed ha py oly ic
suga s (le oglucosan, glucose and xylose) can be con e ed in accep able yields o LA (75, 74 and
61% based on C con en ) using lan hanum i la e, a Lewis acid ca alys [
85
]. This app oach can be
success ully ex ended o cellulose (73% C yield). Fu he p oo o his s a egy is encoun e ed in he
wo k by Li e al., which ocused on he con e sion o hexoses ( om cellulose) and pen oses ( om
hemicellulose) o me hyl lac a e (MeLac) in nea -supe c i ical me hanol and in he p esence o se e al
me al chlo ide ca alys s [
86
]. In he case o glucose, he bes yield o MLA (47%) is ob ained wi h LaCl
3
,
a ca alys ha also ende s good MLA yields om uc ose (64%) and xylose (33%).
Xyla ic acid is an in e media e o glu a ic acid and, hus, o PVC, esins, and syn he ic ubbe s,
as well as se e al ine chemicals and ag ochemicals. The e o e, xyla ic acid has unques ionably been
iden i ied as one o he en key pla o m chemicals om biomass. Ve y ecen ly, Sabuda and Saha
s udied se e al comme cial p ecious me als on ca bon ca alys s o he oxida ion o xylose o xyla ic
acid in wa e , inding ha P /C is he mos ac i e ca alys . High oxygen p essu es and ca alys amoun s
acili a e deepe oxida ion, hence educing he p esence o he in e media e xylonic acid and inc easing
he yield o xyla ic acid, he end p oduc [87].
Finally, like o he pen oses, xylose can be employed as a monome o as a sou ce o monome s.
S a ing om xylose and using he ca aly ic ac i i y o in-con aining silica es, Ellio e al. ha e c ea ed
a new monome o unc ional polyes e s: ans-2,5-dihyd oxy-3-pen enoic acid me hyl es e (DPM).
Wi h Sn-Be a as he bes ca alys and unde op imized condi ions, he au ho s eached a 33% yield
o DPM [
88
]. Mo eo e , Lopez-Vidal e al. de i ed new xan ha e (XAN) and hionoca bona e (TOC)
cyclic monome s om 2-deoxy-D- ibose and D-xylose. A subsequen ing-opening polyme iza ion
(ROP) o hese monome s ende ed polyca bona es. To each he monome s, CS
2
and CO
2
eac ed
wi h he suga s h ough a cycloca bona ion p ocess, and u he eac ion wi h mesyl chlo ide wi h
ime hylamine esul ed in he cyclic xan ha e [89].
Ca alys s 2018,8, 637 32 o 39
Table 9. Summa y o he de ails o wo ks ound in li e a u e dealing wi h he ans o ma ion o mic o and mac oalgal biomass.
Reac ion and Biomass Sou ce P oduc O he P oduc s Ca alys Reac ion Condi ions Main Resul s Re e ence
Acid hyd olysis
Nanochlo opsis salina Monosaccha ides
Fo mic acid
Le ulinic acid
5-HMF
Fu u al
HCl
H2SO4
T = 90 ◦C
Cbiomass = 10% w/ in 10 mL H2O
Cca = 10% w/w
= 60 min
Ymal = 90%
TOF = 0.6 gmal molca −1h−1
[95]
Acid hyd olysis
Chlo ella ulga is Malodex in H2SO4
T = 90 ◦C
Cca boh = 37.3%
Cca = 0.56 M
=5h
Ymonosac = 243 mg/g TOF = 0.052
molsuga gca −1h−1
[96]
Acid hyd olysis
Chlo ella sp. and
Nanochlo opsis gadi ana
Le ulinic es e H2SO4
T = 130 ◦C
Cbiomass = 38 g/L alcohol
Cglucose = 6.8–28.1% w/w
Cca = 15% w/
=2h
YLe ulina e = 40%
TOF = 0.0252 molle molca −1h−1
[97]
Chemo-enzyma ic hyd olysis
Dunaliella e iolec a Glucose
H2SO4
α-amylase and
α-glucosidase
T = 50–90 ◦C
Cpolyssac = 0.1–1 g/20–100 mL
Cca = 0.5–1.5% Acid ca alys
T = 37 ◦C
=24h
YMF = 90%
TOF = 0.136 molgluc molca −1h−1[98]
Chemo-enzyma ic hyd olysis
Dunaliella e iolec a Glucose E hanol HCl and H2SO4
Saccha omyces ce e isiae
T = 121 ◦C
Cbiomass = 5% w/
Cca = 0.05–1M Acid ca alys
Cenzyme = 0.1–1.0 mL/g
T = 35–55 ◦C
pH = 3.5–6.5
= 15 min
Ysuga = 42.0%
Csuga = 21 mg/mL
Ce hanol = 0.44 g/g glucose
TOF = 0.467 molglu molca −1h−1
[99]
Chemo-enzyma ic hyd olysis
G acila ia e ucosa
Glucose
Galac ose
3,6-anhyd o Galac ose
Le ilinic acid
5-HMF
E hanol
HCl
H2SO4
Cellic C ec2
T = 125 ◦C
Cbiomass = 2% w/w
Cca = 0.01–1.5 N Acid ca alys
Cenzyme = 150 FPU/mL
T = 50 ◦C
pH = 5
= 60 min
Xca bohyd = 57.2%
Ysuga = 21.3–37.4%
TOF = 0.039 molmonosac molca −1
h−1
[100]
Chemo-enzyma ic hyd olysis
Scenedesmus sp. Monosaccha ides E hanol HCl
Viscozyme L
T = 121 ◦C
Cbiomass = 2% w/
Cca = 0.5 M Acid ca alys
Cenzyme = 20 FBGU/g biomass
T = 45 ◦C
pH = 5.5
= 45 min
Ysuga = 37.9% (HCl)
TOF = 0.098 mol
monosac
mol
ca −1
h
−1
=72h
Ysuga = 43.4% (Enz.)
[101]
Enzyma ic hyd olysis
G ea Sal lake USU080 Lac ic acid Lac obacillus casei 12A
T = 37 ◦C
Cbiomass = 15% w/
Cenzyme = 1% /
200–250 pm
= 3–24 h
Ylac ic = 11.7g/L [102]
Enzyma ic hyd olysis
Nanochlo opsis oceanic 2,3-Bu anediol Klebsiella oxy oca
T = 37 ◦C
Cbiomass = 15% w/
Csuga = 5 g/L
150 pm
= 6–7 h
Y2,3-BDO = 0.31 g/g suga s
0.0031 mol2,3-BDO L−1h−1
[103]

Ca alys s 2018,8, 637 33 o 39
Table 9. Con .
Reac ion and Biomass Sou ce P oduc O he P oduc s Ca alys Reac ion Condi ions Main Resul s Re e ence
Enzyma ic hyd olysis
Scenedesmus obliquus
Monosaccha ides
(Glucose and xylose) O ganic acids Celluclas 1.5L No ozyme 188
Alkaline-pe oxide p e ea men
T = 50 ◦C
Cbiomass = 6% w/w
Cenzymes = 10FPU/g
and 20CBU/g
pH = 4.9
300 pm
= 6–7 h
Ysug s = 0.098 g/g biomass [104]
Ca aly ic-hyd o he mal p ocess
Kappaphycus al a ezzi
Glucose
Galac ose
Le ulinic acid
5-HMF
Fu u al
H2SO4
T = 160–175 ◦C
Cbiomass = 2 g/30 mL
Cca = 1% w/w
= 20 min
Ymonosac = 14.5 g/L
TOF = 0.364 molmonosac gca −1h−1
[105]
Ca aly ic-hyd o he mal p ocess
En e omo pha in es inalis
Glucose
Galac ose
Xylose
Mannose
Le ulinic acid
5-HMF
Fu u al
H2SO4T = 156 ◦C
Cca = 1.3% w/w
= 11 min
Ymonosac = 28.6% [106]
Ca aly ic-hyd o he mal p ocess
Algina e om mac oalgae and
cellulose
H+medium: Fu u al, Mannu onic, Gulu onic acids
-OH medium:
Lac ic, Fuma ic, Malic acids
HCl
NaOH
T = 150 ◦C
Calgina e = 20 g/L
pH = 1–13
= 30 min
Ymonome s = 43% in acid medium
TOF = 0.955 molmonom molca −1h−1
[107]
Ca aly ic-hyd o he mal
lique ac ion p ocess
Chlo ella ulga is
Bi-oil
Cyclic ke ones
Lac ones
Fu ans
Phenols
H2SO4
CH3CO2H
T = 220–330 ◦C
Cbiomass = 100 g/L
in H2O
Cca = 0.01–0.1 mol/L
= 30 min
Yp oduc s = 16% [108]
Ca alys s 2018,8, 637 34 o 39
7. Conclusions and Fu u e Pe spec i e
Se e al ca aly ic s a egies a e p esen ly being de eloped om biomass. A i s app oach in ol es
py olysis o gasi ica ion o liquid and gases ha a e u he sepa a ed and ans o med in o chemicals.
Secondly, ca alysis can be di ec ly applied o plan o algal biomass o i s main polyme s, namely, s a ch,
cellulose, lignin, and hemicelluloses. A inal s a egy is he de elopmen o ca aly ic ou es om he
monome s cons i u ing hese polyme s, such as glucose, uc ose, xylose, and mannose, among o he s.
Conce ning he wo la e s a egies, he e has been an o e whelming amoun o scien i ic and echnical
in o ma ion epo ed in ecen decades. Ne e heless, se e al hind ances emain, in pa icula he
p esence o a conside able a ie y and concen a ion o by-p oduc s, due o he p esence o se e al
side- eac ions and he decomposi ion o u he ans o ma ion o a ge p oduc s. In hese ega ds,
wo k on glucose o xylose o u ans by dehyd a ion using mixed B øns ed and Lewis ca alys s is
eaching e y high yields o main p oduc s ei he di ec ly om hese monome s and, mo e ecen ly,
om cellulose and, mos e iden ly, om xylose- ich hemicelluloses. I is wo hwhile highligh ing
ha he mos adequa e esul s a e ob ained when combining eac ion wi h sepa a ion echniques,
mainly ex ac ion, o a oid u he deg ada ion o 5-HMF o u u al, ypically in liquid-liquid o
solid-liquid-liquid sys ems. Fo solid ca alysis, epime iza ion, isome iza ion and hyd ogena ion
combina ions in in-se ies ixed bed eac o s show he adequa e way o combine ans o ma ions and
each highe yields o he p oduc s o in e es . The e o e, challenges posed by selec i i y (low yields)
s ill emain o se e al p oduc s ha need deep ans o ma ion o he C5 and C6 eac an s, equi ing
mo e speci ic ca alys s ha educe he impac o side- eac ions. In he e en ha by-p oduc s we e o
in e es in hese cases, he combina ion o eac ion and/o sepa a ion ope a ions would be o in e es
o use he e. In se e al ou es, he s abili y and/o he less-s udied egene a ion capaci y o he ca alys
should be imp o ed o gain economic easibili y in scale-up o p ocesses. Finally, in ecen yea s,
esea che s ha e u ned hei a en ion o o he plen i ul biomass eeds ock: mac o- and mic oalgae.
Though he applica ion o ca alysis o hese is only s a ing, he knowledge gained wi h s a chy and
lignocellulosic biomass is being applied and se e al pla o m chemicals (galac ose, glucose) and a ge
p oduc s (lac ic acid, ke ones, lac ones, and u ans) a e being p oduced.
Au ho Con ibu ions: J.E., P.Y., and M.L. con ibu ed equally o his wo k.
Funding:
Funding om he Spanish Minis y o Science, Inno a ion and Uni e si ies (MISIU) h ough g an
CTQ-2017-84963-C2-1-R is g a e ully acknowledged.
Con lic s o In e es : The au ho s decla e no con lic o in e es .
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